Application Fundamentals

Quickview

Android applications are composed of one or more application components (activities,
services, content providers, and broadcast receivers)

Each component performs a different role in the overall application behavior, and each
one can be activated individually (even by other applications)

The manifest file must declare all components in the application and should also declare
all application requirements, such as the minimum version of Android required and any hardware
configurations required

Non-code application resources (images, strings, layout files, etc.) should include
alternatives for different device configurations (such as different strings for different
languages and different layouts for different screen sizes)

In this document

Android applications are written in the Java programming language. The Android SDK tools compile
the code—along with any data and resource files—into an Android package, an
archive file with an .apk suffix. All the code in a single .apk file is considered
to be one application and is the file that Android-powered devices use to install the
application.

Once installed on a device, each Android application lives in its own security sandbox:

The Android operating system is a multi-user Linux system in which each application is a
different user.

By default, the system assigns each application a unique Linux user ID (the ID is used only by
the system and is unknown to the application). The system sets permissions for all the files in an
application so that only the user ID assigned to that application can access them.

Each process has its own virtual machine (VM), so an application's code runs in isolation from
other applications.

By default, every application runs in its own Linux process. Android starts the process when any
of the application's components need to be executed, then shuts down the process when it's no longer
needed or when the system must recover memory for other applications.

In this way, the Android system implements the principle of least privilege. That is,
each application, by default, has access only to the components that it requires to do its work and
no more. This creates a very secure environment in which an application cannot access parts of
the system for which it is not given permission.

However, there are ways for an application to share data with other applications and for an
application to access system services:

It's possible to arrange for two applications to share the same Linux user ID, in which case
they are able to access each other's files. To conserve system resources, applications with the
same user ID can also arrange to run in the same Linux process and share the same VM (the
applications must also be signed with the same certificate).

An application can request permission to access device data such as the user's
contacts, SMS messages, the mountable storage (SD card), camera, Bluetooth, and more. All
application permissions must be granted by the user at install time.

That covers the basics regarding how an Android application exists within the system. The rest of
this document introduces you to:

The core framework components that define your application.

The manifest file in which you declare components and required device features for your
application.

Resources that are separate from the application code and allow your application to
gracefully optimize its behavior for a variety of device configurations.

Application Components

Application components are the essential building blocks of an Android application. Each
component is a different point through which the system can enter your application. Not all
components are actual entry points for the user and some depend on each other, but each one exists
as its own entity and plays a specific role—each one is a unique building block that
helps define your application's overall behavior.

There are four different types of application components. Each type serves a distinct purpose
and has a distinct lifecycle that defines how the component is created and destroyed.

Here are the four types of application components:

Activities

An activity represents a single screen with a user interface. For example,
an email application might have one activity that shows a list of new
emails, another activity to compose an email, and another activity for reading emails. Although
the activities work together to form a cohesive user experience in the email application, each one
is independent of the others. As such, a different application can start any one of these
activities (if the email application allows it). For example, a camera application can start the
activity in the email application that composes new mail, in order for the user to share a picture.

An activity is implemented as a subclass of Activity and you can learn more
about it in the Activities
developer guide.

Services

A service is a component that runs in the background to perform long-running
operations or to perform work for remote processes. A service
does not provide a user interface. For example, a service might play music in the background while
the user is in a different application, or it might fetch data over the network without
blocking user interaction with an activity. Another component, such as an activity, can start the
service and let it run or bind to it in order to interact with it.

A service is implemented as a subclass of Service and you can learn more
about it in the Services developer
guide.

Content providers

A content provider manages a shared set of application data. You can store the data in
the file system, an SQLite database, on the web, or any other persistent storage location your
application can access. Through the content provider, other applications can query or even modify
the data (if the content provider allows it). For example, the Android system provides a content
provider that manages the user's contact information. As such, any application with the proper
permissions can query part of the content provider (such as ContactsContract.Data) to read and write information about a particular person.

Content providers are also useful for reading and writing data that is private to your
application and not shared. For example, the Note Pad sample application uses a
content provider to save notes.

A content provider is implemented as a subclass of ContentProvider
and must implement a standard set of APIs that enable other applications to perform
transactions. For more information, see the Content Providers developer
guide.

Broadcast receivers

A broadcast receiver is a component that responds to system-wide broadcast
announcements. Many broadcasts originate from the system—for example, a broadcast announcing
that the screen has turned off, the battery is low, or a picture was captured.
Applications can also initiate broadcasts—for example, to let other applications know that
some data has been downloaded to the device and is available for them to use. Although broadcast
receivers don't display a user interface, they may create a status bar notification
to alert the user when a broadcast event occurs. More commonly, though, a broadcast receiver is
just a "gateway" to other components and is intended to do a very minimal amount of work. For
instance, it might initiate a service to perform some work based on the event.

A unique aspect of the Android system design is that any application can start another
application’s component. For example, if you want the user to capture a
photo with the device camera, there's probably another application that does that and your
application can use it, instead of developing an activity to capture a photo yourself. You don't
need to incorporate or even link to the code from the camera application.
Instead, you can simply start the activity in the camera application that captures a
photo. When complete, the photo is even returned to your application so you can use it. To the user,
it seems as if the camera is actually a part of your application.

When the system starts a component, it starts the process for that application (if it's not
already running) and instantiates the classes needed for the component. For example, if your
application starts the activity in the camera application that captures a photo, that activity
runs in the process that belongs to the camera application, not in your application's process.
Therefore, unlike applications on most other systems, Android applications don't have a single entry
point (there's no main() function, for example).

Because the system runs each application in a separate process with file permissions that
restrict access to other applications, your application cannot directly activate a component from
another application. The Android system, however, can. So, to activate a component in
another application, you must deliver a message to the system that specifies your intent to
start a particular component. The system then activates the component for you.

Activating Components

Three of the four component types—activities, services, and
broadcast receivers—are activated by an asynchronous message called an intent.
Intents bind individual components to each other at runtime (you can think of them
as the messengers that request an action from other components), whether the component belongs
to your application or another.

An intent is created with an Intent object, which defines a message to
activate either a specific component or a specific type of component—an intent
can be either explicit or implicit, respectively.

For activities and services, an intent defines the action to perform (for example, to "view" or
"send" something) and may specify the URI of the data to act on (among other things that the
component being started might need to know). For example, an intent might convey a request for an
activity to show an image or to open a web page. In some cases, you can start an
activity to receive a result, in which case, the activity also returns
the result in an Intent (for example, you can issue an intent to let
the user pick a personal contact and have it returned to you—the return intent includes a
URI pointing to the chosen contact).

For broadcast receivers, the intent simply defines the
announcement being broadcast (for example, a broadcast to indicate the device battery is low
includes only a known action string that indicates "battery is low").

The other component type, content provider, is not activated by intents. Rather, it is
activated when targeted by a request from a ContentResolver. The content
resolver handles all direct transactions with the content provider so that the component that's
performing transactions with the provider doesn't need to and instead calls methods on the ContentResolver object. This leaves a layer of abstraction between the content
provider and the component requesting information (for security).

The Manifest File

Before the Android system can start an application component, the system must know that the
component exists by reading the application's AndroidManifest.xml file (the "manifest"
file). Your application must declare all its components in this file, which must be at the root of
the application project directory.

The manifest does a number of things in addition to declaring the application's components,
such as:

Identify any user permissions the application requires, such as Internet access or
read-access to the user's contacts.

Declare the minimum API Level
required by the application, based on which APIs the application uses.

Declare hardware and software features used or required by the application, such as a camera,
bluetooth services, or a multitouch screen.

API libraries the application needs to be linked against (other than the Android framework
APIs), such as the Google Maps
library.

And more

Declaring components

The primary task of the manifest is to inform the system about the application's components. For
example, a manifest file can declare an activity as follows:

In the <application>
element, the android:icon attribute points to resources for an icon that identifies the
application.

In the <activity> element,
the android:name attribute specifies the fully qualified class name of the Activity subclass and the android:label attributes specifies a string
to use as the user-visible label for the activity.

Activities, services, and content providers that you include in your source but do not declare
in the manifest are not visible to the system and, consequently, can never run. However,
broadcast
receivers can be either declared in the manifest or created dynamically in code (as
BroadcastReceiver objects) and registered with the system by calling
registerReceiver().

Declaring component capabilities

As discussed above, in Activating Components, you can use an
Intent to start activities, services, and broadcast receivers. You can do so
by explicitly naming the target component (using the component class name) in the intent. However,
the real power of intents lies in the concept of intent actions. With intent actions, you simply
describe the type of action you want to perform (and optionally, the data upon which you’d like to
perform the action) and allow the system to find a component on the device that can perform the
action and start it. If there are multiple components that can perform the action described by the
intent, then the user selects which one to use.

The way the system identifies the components that can respond to an intent is by comparing the
intent received to the intent filters provided in the manifest file of other applications on
the device.

When you declare a component in your application's manifest, you can optionally include
intent filters that declare the capabilities of the component so it can respond to intents
from other applications. You can declare an intent filter for your component by
adding an <intent-filter> element as a child of the component's declaration element.

For example, an email application with an activity for composing a new email might declare an
intent filter in its manifest entry to respond to "send" intents (in order to send email). An
activity in your application can then create an intent with the “send” action (ACTION_SEND), which the system matches to the email application’s “send”
activity and launches it when you invoke the intent with startActivity().

Declaring application requirements

There are a variety of devices powered by Android and not all of them provide the
same features and capabilities. In order to prevent your application from being installed on devices
that lack features needed by your application, it's important that you clearly define a profile for
the types of devices your application supports by declaring device and software requirements in your
manifest file. Most of these declarations are informational only and the system does not read
them, but external services such as Google Play do read them in order to provide filtering
for users when they search for applications from their device.

For example, if your application requires a camera and uses APIs introduced in Android 2.1 (API Level 7), you should declare these as
requirements in your manifest file. That way, devices that do not have a camera and have an
Android version lower than 2.1 cannot install your application from Google Play.

However, you can also declare that your application uses the camera, but does not
require it. In that case, your application must perform a check at runtime to determine
if the device has a camera and disable any features that use the camera if one is not available.

Here are some of the important device characteristics that you should consider as you design and
develop your application:

Screen size and density

In order to categorize devices by their screen type, Android defines two characteristics for
each device: screen size (the physical dimensions of the screen) and screen density (the physical
density of the pixels on the screen, or dpi—dots per inch). To simplify all the different
types of screen configurations, the Android system generalizes them into select groups that make
them easier to target.

By default, your application is compatible with all screen sizes and densities,
because the Android system makes the appropriate adjustments to your UI layout and image
resources. However, you should create specialized layouts for certain screen sizes and provide
specialized images for certain densities, using alternative layout resources, and by declaring in
your manifest exactly which screen sizes your application supports with the <supports-screens> element.

Many devices provide a different type of user input mechanism, such as a hardware keyboard, a
trackball, or a five-way navigation pad. If your application requires a particular kind of input
hardware, then you should declare it in your manifest with the <uses-configuration> element. However, it is rare that an application should require
a certain input configuration.

Device features

There are many hardware and software features that may or may not exist on a given
Android-powered device, such as a camera, a light sensor, bluetooth, a certain
version of OpenGL, or the fidelity of the touchscreen. You should never assume that a certain
feature is available on all Android-powered devices (other than the availability of the standard
Android library), so you should declare any features used by your application with the <uses-feature>
element.

Platform Version

Different Android-powered devices often run different versions of the Android platform,
such as Android 1.6 or Android 2.3. Each successive version often includes additional APIs not
available in the previous version. In order to indicate which set of APIs are available, each
platform version specifies an API Level (for example, Android 1.0 is API Level
1 and Android 2.3 is API Level 9). If you use any APIs that were added to the platform after
version 1.0, you should declare the minimum API Level in which those APIs were introduced using the
<uses-sdk>
element.

It's important that you declare all such requirements for your application, because, when you
distribute your application on Google Play, the store uses these declarations to filter which
applications are available on each device. As such, your application should be available only to
devices that meet all your application requirements.

For more information about how Google Play filters applications based on these (and other)
requirements, see the Filters on Google Play
document.

Application Resources

An Android application is composed of more than just code—it requires resources that are
separate from the source code, such as images, audio files, and anything relating to the visual
presentation of the application. For example, you should define animations, menus, styles, colors,
and the layout of activity user interfaces with XML files. Using application resources makes it easy
to update various characteristics of your application without modifying code and—by providing
sets of alternative resources—enables you to optimize your application for a variety of
device configurations (such as different languages and screen sizes).

For every resource that you include in your Android project, the SDK build tools define a unique
integer ID, which you can use to reference the resource from your application code or from
other resources defined in XML. For example, if your application contains an image file named logo.png (saved in the res/drawable/ directory), the SDK tools generate a resource ID
named R.drawable.logo, which you can use to reference the image and insert it in your
user interface.

One of the most important aspects of providing resources separate from your source code
is the ability for you to provide alternative resources for different device
configurations. For example, by defining UI strings in XML, you can translate the strings into other
languages and save those strings in separate files. Then, based on a language qualifier
that you append to the resource directory's name (such as res/values-fr/ for French string
values) and the user's language setting, the Android system applies the appropriate language strings
to your UI.

Android supports many different qualifiers for your alternative resources. The
qualifier is a short string that you include in the name of your resource directories in order to
define the device configuration for which those resources should be used. As another
example, you should often create different layouts for your activities, depending on the
device's screen orientation and size. For example, when the device screen is in portrait
orientation (tall), you might want a layout with buttons to be vertical, but when the screen is in
landscape orientation (wide), the buttons should be aligned horizontally. To change the layout
depending on the orientation, you can define two different layouts and apply the appropriate
qualifier to each layout's directory name. Then, the system automatically applies the appropriate
layout depending on the current device orientation.

For more about the different kinds of resources you can include in your application and how
to create alternative resources for various device configurations, see the Application Resources developer guide.